By introducing twin structures in wurtzite AlN, researchers from the Suzhou Institute of Nano-tech and Nano-bionics (SINANO), Chinese Academy of Sciences have made a breakthrough in the preparation of semipolar (10-11) AlN – which can reduce internal electric field dramatically than the polar c-direction AlN. The team demonstrated that the epitaxial growth of (10-11) semi-polar AlN on c-plane substrates by constructing (10-11) and (10-13) twin structures. This new method is relative feasible than conventional methods. The SINANO team's improved growth method has huge prospect to develop high-quality semi-polar AlN.
At first, researchers observed AlN film which was composed of two layers with different cross-sectional shapes by using TEM (Transmission Electron Microscope). The initial layer looks like irregular sawtooth while the second layer is the columnar structure grown on the sawtooth layer, where distinct boundaries could be seen between neighboring columns. SAED (Selected Area Electron Diffraction) demonstrated that the sawtooth layer and the columnar layer was along [0001] and [10-11] direction, respectively. Combining SAED and HRTEM (High Resolution Transmission Electron Microscope), researchers verified that the sawtooth layer are combined with the upper columns by (10-11) twin boundaries. What’s more, the adjacent columns were combined by (10-13) twin boundaries. To date, however, there were few reports about twin structures in bulk wurtzite materials so far.
Besides the above experiment, the SINANO team has also clarified the nucleation and growth mechanics by ECS (Equilibrium Crystal Shape) theory. The calculation indicated that {10-11} crystal planes become the facets on surface when the growth temperature is less than 1390 °C under the growth conditions used in AlN growth on sapphire of their work. In fact, the actual growth temperature is 1300 °C, which made {10-11} crystals become facets on surface possibly. If AlN continues growing on the previous {1011} facets of the sawtooth layer, c-direction could not keep paralleling the z-axis (parallel with [0001]) any longer. Apparently, once the subsequent crystal changes growth orientation, large-angle grain boundaries must be developed at {1011} facets on surface. The formation of large-angle grain boundaries means that the energy of the crystal will increase significantly. Contrary to the common boundary, the energy of twin boundary is much lower. Actually, the energy of coherent twin boundary is only one-tenth of that of a common one while the value for an incoherent twin boundary is only a half. Therefore, forming twin boundaries at {1011} facets on surface contributes to remaining a low-energy and more stable state of the crystal.
The AlN research team was inspired by the idea of preparing semi-polar AlN thick film by twin growth mode. In order to control the facets on surface precisely, researcher utilized epitaxial lateral overgrowth (ELOG) technology to obtain the desired growth front of AlN on trench patterned substrates. As expected, they prepared the (10-11) semi-polar AlN successfully on the patterned polar 6H-SiC by forming (10-11) or (10-13) twin structure at the facets on the surface.
“Although there are possible twin boundaries defined by (10-11), (10-12) and (10-13) in wurtzite structures, few articles report twin structures in Ⅲ-Ⅴ bulk nitrides” said ZHANG Jicai, SINANO professor at the SINANO Test and Analysis Platform. “We have found (10-11) and (10-13) twin structures in the same AlN thick film.”
"All the time, researchers merely regard twin structures as defects but ignore its intrinsic property of changing crystal orientions. " said XU Ke, the leader of the SINANO Test and Analysis Platform. "We have prepared semipolar (10-11) AlN by utilizing this feature for the first time."
The paper, "Nucleation and growth of (10-11) semi-polar AlN on (0001) AlN by Hydride Vapor Phase Epitaxy " has been published online in the journal Scientific Reports.
Contact information: Prof. ZHANG Jicai & Prof. XU Ke
Suzhou Institute of Nano Tech and Nano Bionics ,Chinese Academy of Science
Suzhou,Jiangsu 215125,China.
E-mail:jczhang2010@sinano.ac.cn, kxu2006@sinano.ac.cn
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